1. Field of the Invention
The present invention relates generally to flowmeters and provers and, more particularly, to a system and methods for proving flowmeters which provide computationally derived readings of fluid flow, such as ultrasonic flowmeters and Coriolis mass flowmeters.
2. Description of the Background
As referred to herein, flowmeters that mechanically integrate the flow profile and produce representative output pulses are referred to as mechanical or non-computational flowmeters, because they contain components that move in response to fluid flow such as turbines, vanes, paddles, or the like, that are mechanically related to the flow. In these meters, a number of turns of a turbine, for instance, may be utilized to determine the flow with minimal or no computations required. Computational flowmeters, as used herein, refers to flowmeters to measure fluid flow without use of mechanically moving elements within the fluid flow path. Examples of computational flowmeters include ultrasonic flowmeters and Coriolis mass flow meters. Meters of this type integrate the flow field through data assimilation and calculation. Both computational and non-computational flowmeters may produce pulses wherein the number of pulses is related to measurement of fluid flow.
Meter provers use a calibrated section of pipe with appropriate starting and ending sensors. A flag or displacer moves between the starting and ending sensors along with a calibrated fluid column. The meter prover provides a means of comparing a known volume (the volume between the detectors) with the reported throughput of the meter.
In many cases, valuable fluid flows through pipelines. In such cases, it is absolutely necessary to verify meter operation at regular intervals so that proper accounting may be made for the volume of fluid that flows through the pipeline. By proving the meters on a regular basis, e.g., monthly, the flowmeter accuracy can be verified so that accounting requirements are satisfied. Flowmeter inaccuracies may be caused by temporary problems due to the passage of contaminants through the line. Flowmeter inaccuracies may also be more of a permanent nature due to flowmeter wear, obstruction, and the like. Wear is especially prominent on the non-computational or mechanical flowmeters. On the other hand, the computational flowmeters have little wear due to the absence of moving mechanical components within the flow stream.
Meter provers are of several types. Large volume meter provers utilize a large calibrated volume and are heavy and bulky. Small volume meter provers utilize a small calibrated volume of fluid. The small volume meter provers are compact and portable and may be readily used in the field to test the flowmeters. Small volume meter provers may often be called dual chronometry provers because these provers utilize two highly accurate clocks to control and process the collection of meter pulses. A representative dual chronometer meter prover 10 is shown in FIG. 1. In this example, region 12 in flow tube 14 between first detector 16 and second detector 18 is the calibrated volume. The process collects only whole pulses produced by the flowmeter and thereby avoids the difficulty of reconciling partial pulses against the prover's collected volume. This is accomplished by starting the pulse collection with the first whole pulse as indicated at 20 after first detector 16 in the prover 10 is tripped by flag or displacer 24. The pulse may be started at the zero crossing, rising, falling edges, selected trigger levels, or the like as desired. Simultaneously, a high speed clock begins timing the duration of the pulse collection as discussed below. This process ends when the leading edge, or other selected portion of the first pulse is encountered as indicated at 22 after the second prover detector 18 is tripped. The meter prover computations result in producing a meter factor K which provides the number of pulses from the flowmeter per unit of volume through the pipeline.
As a practical matter, the small volume meter provers with dual chronometers are often the only economical means for proving flowmeters for pipelines in the field. However, it has been found that the computational flowmeters produce unacceptable variations when tested at different times by dual chronometry small volume provers. The computational flowmeter variations may be decreased to an acceptable level when tested using large volume meter provers. However, the large volume meter provers are not practical for field use where the flowmeters are tested. Due to these problems, computational flowmeters are not always acceptable for use in pipelines for accounting purposes where it is required that the flowmeters be successfully proved.
However, it would be highly desirable and economical to be able to utilize the computational flowmeters in such pipelines due to the fact that computational flowmeters have less wear than the non-computational flowmeters with moving parts. Use of computational flowmeters could provide more reliable measurements without the need to change out flowmeters so frequently. Efforts have been made in the past by those of skill in the art to correct these problems, but solutions have not been found that are acceptable by the pipeline industry.
In more detail, dual chronometry provers utilize two clocks to control the collection of meter pulses so as to collect only whole pulses and avoid the difficulty of reconciling partial pulses against the prover's collected volume. As noted above, meter prover 10 has two spaced apart detectors between which is an exactly calibrated volume 12. Flag or displacer 24 is released for proving and flows with the fluid in flow tube 14. Pulse collection of pulses 26 produced by the flowmeter to be tested is started with the first whole pulse as indicated at 20 produced after first detector 16 in prover 10 is tripped by displacer 24. Simultaneously, a high speed clock begins timing the duration of the pulse collection, i.e.; the time between the points indicated at 20 and 22.
Double chronometry pulse interpolation uses a high frequency master oscillator which increments time very precisely (e.g., 0.000001 parts of a second). This master oscillator operates two counters which may be referred to herein as time counter “A” and time counter “B.” Time counter “A” is started when displacer or flag 24 trips first detector switch 16. Time counter “B” is started with the leading edge of the first flowmeter pulse as indicated at 20 after counter “A” has started. Counter “A” is stopped when displacer or flag 24 trips final detector switch 18. Time counter “B” is stopped with the leading edge of the first flowmeter pulse after counter “A” has stopped as indicated at 22. Using the ratio of the counter time “A” and counter time “B” will allow for accurately counting a fraction of a flowmeter pulse as shown below:K in pulses/unit volume=(Time A in seconds/D in unit volume)×(C pulses/Time B in seconds)where:
K=K-Factor or pulses per unit volume from the flowmeter
A=Time for displacing calibrated volume measured by first time counter
B=Time for whole flowmeter pulses measured by second time counter
C=Total number of whole flowmeter pulses
D=Calibrated volume
Typical Example for 12″ Small Volume Prover:
A=000.58377 seconds as determined from time counter “A”
B=000.58329 seconds as determined from time counter “B”
C=Accumulated whole pulses produced by the tested flowmeter as determined by a pulse counter (the pulse counter may or may not be part of the flowmeter)
D=Known displaced volume between the first time counter and the second time counter (e.g., as determined by water draw certification)
In this example K=0.58377 seconds/0.35714bbl×364 pulses/0.58329 seconds.
Accordingly K=1020.0468 pulses per bbl (or other unit of volume)
The following patents disclose various types of provers.
U.S. Pat. No. 3,273,375, issued Sep. 20, 1966, to Howe, discloses a calibrating barrel. More particularly, the invention relates to an apparatus for calibrating a flow meter. Still more particularly, the invention relates to an improved calibrating barrel for use in an apparatus to calibrate flow meters, the invention being characterized by increased accuracy, economy of construction and dependability.
U.S. Pat. No. 3,580,045, issued May 25, 1971, to Pfrehm, discloses a bidirectional meter prover adapted to be connected to a conduit having a meter arranged therein with a valve means connected to the calibration barrel of the meter prover and to the meter such that the valve means is rapidly shifted for movement of the piston in the meter prover in either direction in the calibration barrel for proving the meter. The meter prover is adapted for proving meters used for metering cryogenic liquids with the cryogenic liquid being introduced into the meter proving system and with the cryogenic liquid being vented as the system is cooled to equilibrium temperature; venting is stopped while introduction of cryogenic liquid is continued until thermal equilibrium is reached. Thereafter, the system is operated to prove the meter at equilibrium temperature.
U.S. Pat. No. 4,372,147, issued Feb. 8, 1983, to Waugh et al, discloses a flow meter prover which includes an outer fluid housing having an inlet and an outlet, a measuring conduit coaxially mounted within the outer housing and having first and second sets of fluid apertures adjacent, respectively, the upstream and downstream ends thereof, a fluid barrier mounted within the annular cavity between the outer housing and the conduit, a controllable piston mounted within the conduit, an actuating rod axially projecting from the downstream side of the piston where the free end of the rod extends through the downstream end of the outer housing, a bypass valve connected between the inlet and outlet of the outer housing, and first and second piston detection switches spaced apart along the length of the measuring conduit. There are provisions for automatically correcting for variations in the dimensions of the measuring conduit due to variations in fluid temperature. The prover also includes apparatus for continuously monitoring the integrity of the piston seals, and may be operated with equal fluid pressure on both sides of the piston.
U.S. Pat. No. 4,475,377, issued Oct. 9, 1984, to Halpine, discloses an apparatus for use in calibrating a meter having an outer tubular housing closed at each end, a reduced diameter and reduced length inner tubular barrel supported within the outer house providing an annular space between the exterior of the barrel and the interior of the housings, two spaced apart ports in the housing communicating with the annular area, an annular flange secured to the interior wall of the housing having an opening therein receiving the barrel and an annular flange between the exterior of the barrel and the interior of the housing—dividing the annular space into two portions, the annular flange being arranged so that the barrel may be easily and expeditiously removed while ensuring that no leakage occurs past the flange, a free piston in the barrel and piston detection switches spaced apart on the barrel providing means of indicating passage of precise amount of gas or liquid through the barrel.
U.S. Pat. No. 4,606,218, issued Aug. 19, 1986, to Chisman, III, discloses a compact bidirectional meter prover mechanism that incorporates a straight calibration barrel having spaced detectors sensing passage of a fluid induced displacer piston. A four-way control valve is connected by flow conduits to the conduit containing the flow meter and is connected by a conduit system to respective extremities of the calibration barrel. Launch valves are interposed between the control valve and the calibration barrel and are selectively positioned to permit metering flow through the calibration barrel only after the control valve has been positioned for directional flow of fluid through the conduit system and the calibration barrel. Thus, the necessity for piston prerun is eliminated and minimum cycle time is permitted. The length of the calibration barrel is also minimized by elimination of piston prerun.
U.S. Pat. No. 4,619,134, issued Oct. 28, 1986, to Bohm et al, discloses a testing device for at least one flow meter installed in a pipeline. The device includes a calibration container connected to the pipeline across a switch-over valve. A measuring piston is guided for reciprocating movement in the container between a starting and a terminal position. The measuring piston supports two signal releasing rings which are spaced apart from the other about a distance which when multiplied by the inner clearance of the calibration container determines a calibration volume of the measuring path of the piston. At least one signal generator is installed in the wall of the calibration container and cooperates in a contactless manner with the first and second signal releasing rings so as to produce start and stop pulses for one measuring cycle. The flow meter is connected to a pulse generator responsive to the flow rate and connected to an evaluation circuit which after completion of a measuring cycle compares the counted pulses with the calibration volume.
U.S. Pat. No. 5,392,632, issued Feb. 28, 1995, to Umeda et al, discloses a small volume prover which is compact and capable of obtaining highly reliable measurement, keeping a highly accurate base volume without being affected by temperature and pressure of fluid to be measured. The prover includes a cylindrical outer housing having a fluid inlet and a fluid outlet spaced apart from each other, a cylindrical measuring conduit having both open ends and first and second sets of fluid ports radially made in a wall thereof and coaxially mounted within the outer housing and an annular wall mounted between the outer housing and the measuring conduit at the position between the measuring conduit open end and the first set of ports thereof to form an upstream annular passage and a downstream annular passage. During proving preparation a piston is restrained by a piston actuator provided in the outlet-side end of the outer housing to permit the fluid to pass through the annular passage. At the time of measurement start, the piston is released to run and a valve actuator provided at the inlet side acts as a slide valve to close the first sets of fluid holes.
The above cited prior art does not provide a solution to the aforementioned problems. As noted above, it has been found that when utilizing the dual chronometer small volume prover, that K when determined at the measurement proving intervals, e.g., monthly or quarterly or the like, has varied out of range for the computational flowmeters or flowmeters with no required moving parts in the flow path. This problem is not observed in functioning non-computational flowmeters or flowmeters with moving parts where the measuring element movements are directly sensed within the flow path as with turbine meters. The condition may attain where devices are subject to delay created by gearing clearances or where ancillary devices such as pulse doublers and electronic linearization of the meter's output are utilized. Consequently, there remains a long felt need for improved methods for proving computational flowmeters utilizing dual chronometer small volume provers. Because those skilled in the art have recognized and attempted to solve these problems in the past, they will appreciate the present invention which addresses these and other problems.